Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique.
Generally speaking, a refinery receives the incoming crude oil and produces a variety of different hydrocarbon products in the following manner. The crude product is initially introduced to a crude tower, where it is separated into a variety of different components including naphtha, diesel, and atmospheric bottoms (those that boil above 650xc2x0 F.).
The atmospheric bottoms from the crude tower is thereafter sent for further processing to a vacuum still, where it is further separated into a heavy vacuum residue stream (e.g. boiling above 1000xc2x0 F.) and vacuum gas oil (VGO) stream (boiling between 650xc2x0 F. and 1000xc2x0 F.). At this point the heavy vacuum residue product can be further treated to remove unwanted impurities or converted into useful hydrocarbon products.
Likewise, the VGO stream is further processed in order to yield a usable hydrocarbon product. This further processing may comprise some conversion of the VGO feedstock to diesel (boiling between 400xc2x0 F. and 650xc2x0 F.) as well as some cleaning hydrotreatment prior to its final processing in a Fluid Catalytic Cracker (xe2x80x9cFCCxe2x80x9d) Unit, where it is converted into gasoline and diesel fuels.
It is at this point in the overall refinery, the hydrotreatment/hydrocracking of the VGO stream, which is the subject of the invention. As mentioned above, hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well-known method of catalytically treating such heavy hydrocarbons to increase their commercial value.
More particularly, the aim of such treatment of these hydrocarbon feedstocks, particularly petroleum vacuum gas oil, may include hydrodesulfurization (HDS), carbon residue reduction (CRR), nitrogen removal (HDN), and specific gravity reduction. Additionally, such hydrocarbon streams may be hydrocracked to convert the feedstream into other lighter valuable products.
xe2x80x9cHeavyxe2x80x9d hydrocarbon liquid streams, and particularly heavy vacuum gas oils and deasphalted oils (DAO), generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organometallic compounds which tend to deactivate catalyst particles during contact by the feedstream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the temperature range of between 212xc2x0 F. to 1200xc2x0 F. (100xc2x0 to 650xc2x0 C.) and at pressures of from 20 to 300 atmospheres.
Generally such hydroprocessing is conducted in the presence of a catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other porous particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurirzation, hydrodenitrification, hydrocracking etc., of heavy vacuum gas oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one more promoter(s) or catalytically active metal(s) (or compound(s) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application.
Additionally, in a modern petroleum refinery, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams of varying amounts of contaminants such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in a vacuum gas oils and DAO""s.
Hydrogenating processes treat the charge in the presence of hydrogen and suitable catalysts. The commercial hydroconversion technologies presently on the market use fixed-bed or ebullated-bed reactors with catalysts generally consisting of one or more transition metals (Mo, W, Ni, Co, etc.) supported on alumina (or equivalent material).
The decision to utilize a fixed-bed or ebullated-bed reactor design is based on a number of criteria including type of feedstock, desired conversion percentage, flexibility, run length, product quality, etc. From a general standpoint, the ebullated-bed reactor was invented to overcome the plugging problems with fixed-bed reactors as the feedstock becomes heavier and the conversion (of vacuum residue) increases. In the ebullated-bed reactor, the catalyst is fluid, meaning that it will not plug-up as is possible in a fixed-bed. The fluid nature of the catalyst in an ebullated-bed reactor also allows for on-line catalyst replacement of a small portion of the bed. This results in a high net bed activity, which does not vary with time.
More specifically, fixed-bed technologies have considerable problems in treating particularly heavy charges containing high percentages of heteroatoms, metals and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and subsequent plugging of the reactor. One could utilize numerous fixed-bed reactors connected in series to achieve a relatively high conversion of such heavy vacuum gas oil or DAO feedstocks, but such designs would be costly and, for certain feedstocks, commercially impractical.
Therefore, as mentioned above, to treat these charges, ebullated-bed technologies have been developed and sold, which have numerous advantages in performance and efficiency, particularly with heavy crudes. This process is generally described in U.S. Pat. No. Re 25,770 to Johanson, incorporated herein by reference.
The ebullated-bed process comprises the passing of concurrently flowing streams of liquids or slurries of liquids and solids and gas through a vertically cylindrical vessel containing catalyst. The catalyst is placed in motion in the liquid and has a gross volume dispersed through the liquid medium greater than the volume of the mass when stationary. This technology is utilized in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils.
A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into motion as the liquid and gas pass upwardly through the bed. The catalyst bed level is controlled by a recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors, along with the liquid which is being hydrogenated, pass through the upper level of catalyst particles into a substantially catalyst-free zone and are removed at the upper portion of the reactor.
In an ebullated-bed process, the substantial amounts of hydrogen gas and light hydrocarbon vapors present rise through the reaction zone into the catalyst-free zone. Liquid is both recycled to the bottom of the reactor and removed from the reactor as net product from this catalyst-free zone. Vapor is separated from the liquid recycle stream before being passed through the recycle conduit to the recycle pump suction. The recycle pump (ebullating pump) maintains the expansion (ebullation) of the catalyst at a constant and stable level. Gases or vapors present in the recycled liquid materially decrease the capacity of the recycle pump as well as reduce the liquid residence time in the reactor and limit hydrogen partial pressure.
Reactors employed in a catalytic hydrogenation process with an ebullated-bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downcomer for recycling liquid from the catalyst-free zone above the ebullated catalyst bed to the suction of a recycle pump to recirculate the liquid through the catalytic reaction zone. Alternatively, the ebullating liquid can be obtained from a vapor separator located just downstream of the reactor or obtained from an atmospheric stripper bottoms. The recycling of liquid serves to ebullate the catalyst bed, maintain temperature uniformity through the reactor and stabilize the catalyst bed.
U.S. Pat. No. 4,684,456 to R. P. Van Driesen et. al. teaches the control of catalyst bed expansion in an expanded-bed reactor and is incorporated herein by reference. In the process, the expansion of the bed is controlled by changing the reactor recycle pump speed. The bed is provided with a number of bed level detectors and an additional detector for determining abnormally high bed (interface) level. The interface level is detected by means of a density detector comprising a radiation source at an interior point within the reactor and a detection source in the reactor wall. Raising or lowering the bed level changes the density between the radiation source and the radiation detector.
Although the two processes differ dramatically, both fixed-bed and ebullated-bed reactors can be utilized to process and convert vacuum gas oil feeds, which have a typical boiling range of between 650xc2x0 F. to 1000xc2x0 F. Fixed-bed reactors have heretofore been mainly used when hydrotreating/hydrocracking a VGO feedstream but have numerous disadvantages including the inability to produce a constant quality (i.e. sulfur content) and quantity feedstream to a FCC Unit.
Although ebullated-bed reactor based processes are generally used for conversion of heavier vacuum residue feedstocks, they are also used to clean or treat a lower boiling point vacuum gas oil feedstock. Moreover, as mentioned above, such processes have numerous advantages over the fixed-bed design that are well known in the art including uniformity of product, reduced processing downtime, lower investment, the ability to provide a constant feedstream to a FCC Unit, etc.
Known ebullated-bed reactor designs for processing heavy vacuum gas oil and deasphalted oil feeds have length-to-diameter ratios (L/D) of approximately 6. For a given volume reactor, the greater the length-to-diameter ratio, the more catalyst that can be put into the reactor. Although there are numerous types of ebullated-bed reactor designs, it would be desirable to have a more efficient and effective ebullated-bed reactor process with improved reactor kinetics for the processing of heavy vacuum gas oil and DAO feeds. This would provide for either a cleaner feedstock to a FCC Unit or a smaller reactor size requirement (i.e. lower investment).
This invention is an improved process having numerous advantages over fixed-bed reactor systems and current ebullated-bed designs for processing vacuum gas oil and DAO feeds. This novel process employs a novel ebullating-bed reactor process having a high length-to-diameter ratio wherein the expansion of the catalyst bed above the settled-bed level is controlled at approximately 20% compared to the 40-50% typically used for ebullated-beds in HVGO and vacuum residue service.
The minimal catalyst bed expansion of 20% is set at the point where on-line catalyst withdrawal is feasible. The resulting recycle (ebullating rate) requirement is substantially reduced and is between 0.67 to 1.5 times the fresh feed rate. The dramatically reduced recycle requirement results in enhanced reactor kinetics of the hydrotreating and hydrocracking of heavy vacuum gas oil and DAO feeds. The enhanced kinetics are a direct result of a closer approach to more desirable plug-flow kinetics. Moreover, it allows the operator of the refinery to maintain a consistent volume and quality of product output that does not vary with time.
The object of this invention is to provide a novel ebullated-bed reactor design for treating heavy vacuum gas oil and deasphalted oil feeds.
It is another object of this invention to provide an ebullated-bed reactor that operates at minimum catalyst bed expansion with a minimal recycle requirement of between 0.67 and 1.5 times the fresh feed rate so as to maximize reaction kinetics and approach plug flow reactor process performance.
It is still a further object of this invention to provide an improved ebullated-bed reactor process for processing vacuum gas oil feedstocks that provides a uniform product quality and production rate not varying with time, and allows for the continuous processing of such feedstreams at various rates as required by the refinery.
It is yet a further object of the invention to provide an ebullated-bed reactor with a greater length-to-diameter ratio enabling high catalyst loading per total reactor volume and enhanced conversion and HDS performance.
Novel features of this invention are the high length-to-diameter ratio of the reactor which results in a more catalytic system and the degree of expansion of the catalyst bed which is minimized such that catalyst withdrawal is feasible while maintaining a stable operation but results in enhanced kinetics. Moreover, the recycle (ebullating) liquid requirement is in the range of 0.67 to 1.5 times the fresh oil feed rate relative to standard ebullating recycles rates, which are in excess of 2-3 times the fresh oil feed rate.
Due to the relatively low recycle ratio, the kinetics in the ebullated-bed are closer to plug-flow (i.e. further from CSTR kinetics where CSTR stands for Continuously Stirred Tank Reactor) and therefore result in enhanced conversion and hydrotreatment (e.g. HDS), particularly for VGO feed at high (greater than 95%) HDS.
Either a hot high-pressure separator liquid or stripper bottoms can be used as the recycle ebullating liquid. If stripper bottoms are utilized, there will be enhanced VGO conversion due to the concentrating effect of the recycle material since it contains a high concentration of 650xc2x0 F.+ material. One negative aspect of utilizing stripper bottoms as ebullating liquid is that they must be pumped from near atmospheric pressure to the relatively high pressure of the reactor.
The process of the invention describes the catalytic ebullated-bed hydrotreating/hydrocracking of heavy gas oil or DAO feedstocks comprising:
a) feeding a heavy vacuum gas oil or DAO feedstock, 80% of said feedstock boiling in the range of 650xc2x0 F. to 1000xc2x0 F., together with hydrogen gas to an ebullated-bed reactor, said ebullated-bed reactor having a length-to-diameter ratio greater than six and a level indicator to indicate the level of expansion of the catalyst bed contained therein;
b) separating the effluent from said ebullated-bed reactor into a gas phase and a liquid phase; and
c) recycling said liquid phase to said ebullated-bed reactor at a rate of between 0.67 and 1.5 times the rate of said heavy vacuum gas oil or DAO feedstock;
wherein steps a-c are performed so as to control the catalyst bed expansion rate within said ebullated-bed reactor of between 15-25% as measured by said level indicator.
More specifically, the invention describes an improved process for processing vacuum gas oil feedstocks boiling between 650xc2x0 F. and 1000xc2x0 F. using an optimized ebullated-bed reactor wherein the improvement comprises: the utilization of an ebullated-bed reactor having a length-to-diameter ratio greater than six and wherein the catalyst bed expansion percentage within said ebullated-bed is controlled to between 15% and 25%.